Aldehydes described above are industrially important as intermediates. For example, they can be used to prepare the corresponding alcohols by hydrogenation, carboxylic acids by oxidation, and amines by aminating hydrogenation. The specified classes of compounds are used as raw materials, inter alia, for the preparation of additives for lubricants or plastics.
The preparation of aldehydes by hydroformylation of olefinically unsaturated compounds is a known reaction which is carried out industrially on a large scale in various ways. The starting materials are customarily terminal olefins which are available in large amounts as refinery products and which are predominantly converted into terminal aldehydes. However, specific applications require aldehydes branched in the .alpha. position to the carbonyl group; such aldehydes can be obtained by hydroformylation of olefins having an internal double bond.
In the processing of crude oils, olefins having an internal double bond are obtained only in limited amounts. However, they can be prepared by isomerization of olefins containing a double bond at the end of the carbon chain. This reaction is influenced by various catalysts. A known example is the isomerizing action of carbonyls of iron, cobalt, and/or nickel, which is reported by Asinger and Berg in Chem. Ber. 88,445 (1955). Manuel in JOC. 27,3941 (1962) reports similar results using iron carbonyls as catalysts. Finally, Asinger, Fell, and Collin in Chem. Ber. 96,716 (1963) describe double bond isomerization under the action of acid, basic, and neutral compounds.
Modern hydroformylation processes use as catalysts rhodium or rhodium compounds which are present in very low concentrations; i.e. in an amount of about 1 to about 20 ppm by weight, based on the olefin in the reaction mixture. To counter poisoning of the catalyst, use is made of very pure starting materials, i.e. synthesis gas and olefin. Under these circumstances, the purification of the reactants is a cost-determining factor in modern oxosynthesis. Therefore, efforts are made, by means of technical process measures or adaptation of the course of the reaction, to avoid complex purification steps without reducing the yield or the quality of the reaction product. In terms of the preparation of aldehydes branched in the .alpha. position, this requirement means that internal olefins obtained by isomerization should be able to be used in the oxosynthesis without complicated additional measures.